1. Technical Field
The present invention relates to a thin-walled synthetic resin container, and intends to provide a thin-walled synthetic resin container capable of effectively avoiding lowering of the rigidity of the container, which tends to be caused by its thin-walled nature, to thereby exhibit a required shape stability of the container.
2. Related Art
Synthetic resin containers, such as PET bottles made of polyethylene terephthalate resin, have been widely used as containers, e.g., for filling therein foods, beverages, cosmetics or medicines since such containers are light in weight and can thus be easily handled, have transparency to exhibit a refined appearance comparable to glass containers, and can be produced at low cost.
This type of synthetic resin container has a relatively low mechanical strength against external forces. Therefore, when the container is gripped at its main body portion for pouring the content out of the container, the container inevitably undergoes deformation at its gripped portion. It is thus a typical countermeasure to appropriately control the container wall thickness and form reinforcing means, e.g., longitudinal ribs, lateral ribs or waists (i.e., circumferential grooves surrounding the main body portion), for improving the resistances of the container to external forces, such as buckling strength and rigidity.
Furthermore, there is an increasing demand for thin-walled (or light-weighted) containers so as to reduce the resin amount to be used per one container from a standpoint of effective utilization of resources and reduction in the amount of wastes, resulting in a situation where the rigidity of the container is inevitably further lowered to deal with such a demand. In this instance, particularly in the case of a container having a polygonal cross-section and formed with a waist, the container tends to be deformed in its cross-section into rhombic shape due to the thin-walled nature of the entire container, when external force is applied to the waist portion in a diagonal direction at the corner of the waist portion. From such a viewpoint, in connection with a waist-formed synthetic resin container, there is a strong demand for a container structure having higher buckling strength and rigidity, and capable of minimizing deformation in terms of its outer shape of the container even when it is made thin-walled.
Meanwhile, synthetic resin containers have a relatively low thermal strength, and particularly, containers made of PET resin (polyethylene terephthalate resin) have a limitation on the filling temperature of contents, which must be not higher than approximately 85 to 87° C. Thus, when the contents at temperatures exceeding such a temperature range is filled into the containers, the containers are inevitably deformed due to heat shrinkage thereof. In this respect, there is known a technology as disclosed in JP 7-67732 B2, for example, for improving the heat resistance of containers by carrying out at least two times of biaxial-stretching blow molding before and after an intermediate heat treatment step, and there is indeed a tendency to raise the allowable filling temperatures of contents.
However, when this type of targeted container is thin-walled (or light-weighted) so as to reduce the used resin amount (for example, when the used resin amount is reduced from approximately 69 grams to 55 grams or less, in the case of a 2-liter container), the lower region of the container main body portion tends to bulge outwardly due to the self-weight (i.e., hydraulic head) of the contents and due to the affection of heat of the contents, thereby making it difficult to retain the initial shape of the container. Such bulging is particularly marked in containers having pressure-reduction absorbing panels, which serve to compensate for the shape deformation of the container due to pressure reduction within the container.
Although it is effective to form lateral ribs on a container main body portion so as to retain the outer shape of the container, the ribs may warp due to affection of heat because the container is thin-walled, thereby failing to effectively exhibit the reinforcing function of the ribs. From such a viewpoint, in connection with a synthetic resin container having an improved heat resistance allowing a hot filling of the contents at a relatively high temperature, there is a strong demand for a container structure having an excellent shape stability capable of retaining the initial shape of the container regardless of its thin-walled structure.